1. Field of the Invention
The present invention relates to ceramic composites or composite ceramic bodies which are particularly suitable as implants, such as artificial tooth roots, artificial bones, percutaneous devices, etc. The present invention also relates to a process for the production of said ceramic composites.
2. Description of Related Art
Currently, a wide variety of ceramic materials, such as calcium phosphate hydroxyapatite, alumina, and zirconia ceramic materials have been predominantly utilized as implant materials, especially for artificial tooth roots and bones.
These ceramic materials can be produced, by drying a ceramic slurry which is produced by a wet process. After drying, the dried ceramic slurry is pulverized to make a ceramic powder which ceramic powder is then molded to any desired shape. Molding is carried out by a conventional method, such as pressure molding and casting molding. The molded products are subsequently dried and calcined or sintered. Alternatively, the ceramic materials can be produced from powdered ceramics synthesized in a dry process. The powdered ceramics, without further treatment, are molded in a conventional manner to a predetermined shape, and calcined. The ceramic materials resulting from these processes generally have a dense structure.
On the other hand, ceramic materials with a porous structure are also produced. These porous ceramic material can be produced, by adding a foaming agent to a slurry of powdered ceramics and foaming the mixture, or by mixing powdered ceramics with a thermally decomposable organic substance. After molding of the mixture into a predetermined shape, the molded products are dried and calcined.
For use as implant materials, dense ceramics are suited because they have satisfactory strengths. However, due to less permeability to humor or body fluids such as blood and the like, the dense ceramics suffer from the problem that, when implanted in a patient's body, they do not adequately bond to the surrounding tissue of the implantation site. In other words, the dense ceramics do not provide enough of an inductive effect. This inductive effect, which refers to the formation of new osseous cells around the implant, is essential for attaining a good bond of the implant material to the surrounding tissue.
Insofar as the inductive effect is concerned, porous ceramics provide satisfactory results due to the passage of, humor, such as blood, through pores of the porous ceramics. As a result of this permeation of the humor, new osseous cells are easily produced in the area surrounding the implantation site, i.e., a good inductive effect is induced at the site. However, these ceramics do not have enough strength to be used as an implant material because of the porous structure thereof.
Therefore, an improved ceramic body which shows the advantageous properties of both dense and porous ceramics, namely, high strength and a good inductive effect, respectively, has been sought in this field.
Heretofore, to obtain a ceramic body having the advantageous properties associated with both dense and porous ceramics, various attempts, including the combined use of dense ceramics and porous ceramics, have been made. A typical method for producing a combined ceramic utilizes a bonding between the dense ceramic and the porous ceramic. This bonding has been attained by using different methods. For example these methods includes use of an adhesive, formation of an interlayer between the two ceramic bodies, and coating of a ceramic material or body with another ceramic material.
The ceramic composites produced according to said bonding methods have disadvantages that must to be solved. Since the presence of an adhesive layer or interlayer which has different properties from the ceramics used is essential to the first two methods, the resulting ceramic composites tend to have reduced safety to the human body and a reduced inductive effect. Further, the coating method has a drawback that the resultant coating of the ceramic material on the ceramic body is thin and therefore uses of the ceramics composites are severly restricted to. Furthermore, all of said bonding methods have drawbacks that the bonding strength obtained is not sufficiently high, and the bonding site therefore exhibits a reduced strength.